Fix triangular lighting artifacts
Triangles are notoriously difficult to shade. Unfortunately, EVERYTHING gets turned into triangles by the renderer before rendering happens. Consequently, long, thin quads are also notoriously difficult to shade... because they're actually triangles once they get rendered.

Firstly, turn any triangles into quads. The renderer has a much easier time knowing which direction the quad should be split so that the triangles get smoothed the right direction, than it does trying to figure out which direction your triangle is supposed to be smoothed (without having come from a quad).

Secondly, make your quads as square as possible. It's not always necessary to make them perfectly square, but if you can add a couple of edge loops to make them shorter, that will probably help.

Lastly, (and I doubt this is the problem, but it's worth checking) recalculate your normals (ctrln or Mesh -> Normals -> Recalculate Outside) to make sure they're all facing the same direction.